1. Field of the Invention
The present invention relates to production of pivalic acid and methyl pivalate. More particularly, the present invention is directed to a method for producing pivalic acid and methyl pivalate by cracking C9 and C13 carboxylic acids to pivalic acid or methyl pivalate.
2. Description of Related Art
Pivalic acid (2,2-dimethylpropanoic acid) is widely used as a starting material for various agrochemical, pharmaceuticals, aroma chemicals, specialty chemicals, and polymer additives.
Current processes produce pivalic acid in yields of approximately 74% pivalic acid or less. The by-products are mainly higher molecular weight oligomeric carboxylic acids with carbon numbers of C9 and C13 and higher. The carboxylic acids are typically separated by phase separation from the boron trifluoride dihydrate catalyst and further purified by distillation resulting in the following components: 
In the neo acid process, a hydrocarbon feed or a compound capable of forming a reactive hydrocarbon intermediate, carbon monoxide and recycled catalyst are fed to a continuous process to manufacture an α,α-branched quaternary carboxylic acid. The term “hydrocarbon feed” as used herein includes, but is not limited to, olefins, alcohols and ethers which are capable of forming a reactive hydrocarbon intermediate. The catalyst is a strong acid, typically hydrates of boron trifluoride and occasionally containing a co-catalyst such as phosphoric acid. Other possible catalysts include Lewis acids, zeolite or modified zeolite catalysts, hydrofluoric acid, sulfuric acid, and solid acid catalysts.
One such conventional process for the preparation of pivalic acid is set forth in U.S. Pat. No. 5,241,112 (Sanderson et al.), which is incorporated herein by reference. Sanderson et al. specifically relate to a process for the production of pivalic acid from branched olefins, particularly isobutene and carbon monoxide using a solid acid catalyst and, optionally, with minor amounts of a Lewis acid, such as boron trifluoride. The solid acid catalyst is typically a compound comprising sulfonic acid and phosphonic acid groups, where the phosphonic acid groups are covalently bonded to a polymeric chain.
An example of a typical process for manufacturing pivalic acid is shown in FIG. 1. In reaction step I, conduits 2 and 3 continuously feed isobutylene and carbon monoxide, respectively, to a back mixed, stirred tank reactor 5 having a boron trifluoride dihydrate catalyst disposed therein. Step II, the catalyst recovery step, occurs when the reactor effluent is contacted with water 1 in separator 6 to provide crude pivalic acid, and to extract the boron trifluoride dihydrate catalyst, which is recycled back via conduit 4 to reactor 5. The crude pivalic acid product is fed to distillation tower via conduit 10, where it is then purified in distillation step III. Pivalic acid is separated from the C9 and C13 acids and oligomeric acids with more than 13 carbons. The pivalic acid is removed from tower 7 via conduit 21 and the oligomeric acids are removed as bottoms via conduit 23. C9 and C13 acids are separated from pivalic acid and oligomeric acids with more than 13 carbons via a conduit 22. Depending on a variety of process conditions, significant amounts of C8 and C12 oligomers are also formed from the oligomerization of the feed olefin before carboxylation occurs. Increasing the reaction temperature reduces these unwanted by-products, but also undesirably increases the rate of corrosion. Since these by-product acids occur with carbon numbers that differ by four, they are easily separated by distillation. However, under low temperature conditions, which is preferred to minimize corrosion, selectivity to the desired pivalic acid product is adversely affected.
The present invention addresses these problems and is directed to a novel and unique process for increasing the yield by back cracking oligomeric carboxylic acids to pivalic acid or methyl pivalate. The present invention is able to increase the yield by separating the C9 and C13 acids from the crude pivalic acid product and either recycling them back to the pivalic acid reactor or by post-treatment. These C9 and C13 acids break down to form pivalic acid (C5), thus increasing the effective selectivity. Using the novel approach according to the present invention, lower temperatures can be used to achieve selectivity to pivalic acid, which was heretofore only attainable at higher, more corrosive temperatures. An additional benefit of the process according to the present invention is that the recycled C9 and C13 acid stream acts as a diluent in the feed isobutylene, which further inhibits oligomerization of the isobutylene.